1. Field of the Invention
The present invention relates to a method of manufacturing a connecting rod which connects a piston and a crank shaft together in an engine.
2. Description of Related Art
A connecting rod for an engine has a large-end portion into which is inserted a crank pin which is formed in a crank shaft, a small-end portion into which is inserted a piston pin to be mounted on a piston, and a rod portion which lies between the large-end portion and the small-end portion. A rod main body which is made up by integrally forming the small-end portion, the rod portion, and that half part of the large-end portion which is on the side of the rod portion is connected to a large-end cap which forms the remaining half part of the large-end portion, whereby a connecting rod is assembled.
As a conventional method of manufacturing a connecting rod, there is known the following. Namely, a connecting rod blank which has a large-end potion, a small-end portion, and a rod portion between the small-end and the large-end portion and which is shaped into an integral combination of the rod main body and the large-end cap is manufactured by casting. The connecting rod blank is then divided into the rod main body and the large-end cap (see Japanese Published Unexamined Patent Application No. 195408/1988). However, the connecting rod manufactured by casting has a tendency of being insufficient in strength. As a solution, in a connecting rod in which a high strength is required, the rod main body and the large-end cap are ordinarily manufactured separately by forging. In this conventional method, however, the control of accuracy in the connecting or coupling surfaces of the rod main body and the large-end cap becomes troublesome and a cost reduction becomes difficult.
In Japanese Published Unexamined Patent Application No. 39439/1984, the following method is known. Namely, at the time of cold-coining a forged product which is manufactured by hot-striking, excess metal which is left in a portion to be pierced with a hole is punched out to thereby form a pierced product. It is considered to form the following connecting rod blank by utilizing this art. Namely, a connecting rod blank which has not formed therein holes for the large-end portion and a small-end portion is hot-forged. Then, at the time of cold-coining, a hole is punched in the large-end portion and the small-end portion, respectively, to thereby form a connecting rod blank.
In this method, however, the forming of the entire shape of the connecting rod blank is performed in a hot treatment. Therefore, the step of removing the scales which occur at the time of hot-forging is required and, further, a heating apparatus become necessary. As a result, the cost of manufacturing the connecting rod cannot be reduced to a large extent.
The applicants of this application have earlier proposed the following cold-forged steel in Japanese Patent Application No. 303400/1996. Namely, the steel in question contains in percent by weight (wt %) the following component composition, i.e., 0.05-0.25% of carbon (C), 0.35-0.60% of silica (Si), 0.20-1.30% of manganese (Mn), 0.15-0.60% of chromium (Cr), and further contains one or more selected from the group consisting of 0.030-0.100% of sulfur (S), 0.005-0.040% of tellurium (Te), 0.03-0.30% of lead (Pb), 0.03-0.20% of bismuth (Bi), 0.0005-0.0050% of calcium (Ca), the remaining part being iron (Fe), and also meets the following equations
19xe2x89xa685(%C)+9(%Si)+3(%Mn)+5(%Cr)xe2x89xa625xe2x80x83xe2x80x83(1)
100(%C)+3(%Si)+3(%Mn)xe2x88x929(%Cr)+100(%S)xe2x88x9220(%Te)xe2x89xa623xe2x80x83xe2x80x83(2)
This steel is superior in the workability in cold forging and in fatigue strength, so that it is suitable for forming mechanical parts by cold forging.
In view of the above points, the present invention has an object of providing a method of manufacturing a connecting rod in which the connecting rod blank which is shaped into an integral combination of a rod main body and a large-end cap is cold-forged in a single pressing cycle to thereby largely reduce the cost of manufacturing the connecting rod.
In order to attain the above and other objects, the present invention is a method of manufacturing a connecting rod for an engine, the connecting rod comprising a rod main body and a large-end cap. The method comprises the steps of: forming, by cold forging, a connecting rod blank which is shaped into an integral combination of a rod main body and a large-end cap and which has a large-end portion, a small-end portion, and a rod portion therebetween; and splitting the connecting rod blank into the rod main body and the large-end cap. The step of forming the connecting rod blank is performed by a double-action type of forging apparatus comprising: an upper die which is movable up and down; a lower die; a large-end portion punch which is respectively provided in each of the dies so as to be movable therethrough and to lie opposite to each other at a portion to pierce the large-end portion of the connecting rod blank; and a small-end portion punch which is respectively provided in each of the dies so as to be movable therethrough and so as to lie opposite to each other at a portion to pierce the small-end portion of the connecting rod blank. The step of forming the connecting rod blank comprises: a first forging step of closing the upper die toward the lower die while forging a preform having a shaft portion which corresponds to the rod portion of the connecting rod blank, that large-end mass portion on one end of the shaft portion which corresponds to the large-end portion of the connecting rod blank, and that small-end mass portion of the connecting rod blank which corresponds to the small-end portion of the connecting rod blank, to thereby roughly form the preform into the shape of the connecting rod blank; a second forging step of protruding each of the large-end portion punches and each of the small-end portion punches, respectively, into a forming space between both the dies while forging those parts of the large-end portion and the small-end portion which are to be pierced, to thereby fill underfills inside the forming space; and a piercing step of protruding the large-end portion punch and the small-end portion punch which are provided in one of the upper and lower dies until they reach the other of the dies while pushing out, from the forming space, the large-end portion punch and the small-end portion punch which are provided in the other of the upper and lower dies, to thereby punch out pads remaining in those parts of the large-end portion and the small-end portion which are to be pierced. The first and second forging steps as well as the piercing step are carried out in a single pressing cycle.
According to this invention, the connecting rod blank is split into the rod main body and the large-end cap after the connecting rod blank has been formed. Therefore, special control of accuracy in the connecting surfaces of the rod main body and the large-end cap is not required any more, with the result that the cost of manufacturing the connecting rod becomes smaller. Further, those parts of the large-end portion and the small-end portion which are to be pierced or punched with a hole, respectively, are forged with the large-end portion punch and the small-end portion punch, respectively, in the second forging step. In this manner, it becomes possible to push the material of the preform into underfills (or underfilled parts) which are likely to occur in the cold forging, to thereby fill the underfills with the material. The accuracy in shape of the connecting rod can thus be secured. Thereafter, the pads left in the parts to be pierced is finally punched out by the large-end portion punch and the small-end portion punch which are provided in one of the upper and lower dies. It is thus possible to form by cold forging the connecting rod blank in one pressing cycle with a good accuracy. This results in a large reduction in the cost of manufacturing the connecting rod.
That part of the connecting rod blank which corresponds to the large-end cap is split or divided after the connecting rod blank has been formed. In this case, it is preferable to use, as a large-end portion punch provided in one of the dies, a punch having those projections on an outer periphery of the punch which coincide with split surfaces of the rod main body and the large-end cap of the connecting rod. It is then possible to form, in the piercing step, splitting notches on an inner periphery of a hole in the large-end portion. It is thus not necessary to form the splitting notches in a subsequent processing step, resulting in a further reduction in the manufacturing cost.
In the above-described piercing step, if an oil chamber which forces those large-end portion punch and the small-end portion punch which are provided in the other of the dies toward the forming space is opened to the atmosphere so that each of the punches is pushed out of the forming space in a load-free state, cracks sometimes occur to the hole part of the connecting rod blank. The occurrence of cracks is particularly evident in that part of the peripheral wall of the hole of the small-end portion which is closer to the edge as seen in the direction of punching out the pads. In order to prevent the occurrence of this kind of cracks, preferably the piercing step is performed in a state of applying a load to the large-end portion punch and the small-end portion punch provided on the other of the dies in a direction to resist the pushing of the punches out of the forming space.
In the method as described in the above-described Japanese Published Unexamined Patent Application No. 195408/1988, the following steps are employed. Namely, the connecting rod blank has been forged, the connecting rod blank is split into the rod main body and the large-end cap in a state in which the connecting rod blank is subjected to a cold-temperature or cryogenic treatment. In this invention, it is also considered to split the connecting rod blank in a similar manner after the connecting rod blank has been formed in cold forging. According to this method, due to the cryogenic treatment, the connecting rod blank is fractured or broken through brittleness by a relatively small load, whereby the connecting rod blank is split into the rod main body and the large-end cap. The split surfaces of the rod main body and the large-end cap are brittleness-fractured surfaces, having projections and recessions. By the engagement of these projections and recessions, the accuracy of connecting or assembling of the rod main body and the large-end cap can be secured. However, there will sometimes be formed minute projections which are superimposed on the above-described projections and recessions. When the large-end cap is connected to the rod main body, the minute projections are broken to thereby get included into the space between the split surfaces of the rod main body and the large-end cap. As a result, it sometimes becomes impossible to accurately connect the large-end cap to the rod main body. In such a case, it is preferable to brush the split surfaces of the rod main body and the large-end cap after the splitting. The minute projections on the split surfaces can thus be removed. As a result, the accuracy of assembling the rod main body and the large-end cap can be prevented from becoming poor due to the inclusion of the minute projections into the split surfaces. It is considered to perform the brushing right after splitting the connecting rod blank. However, right after the splitting, the split surfaces are so low in temperature that the vapor in the atmosphere gets adhered thereto by freezing. The minute projections cannot therefore be successfully removed. It is therefore preferable to perform the brushing after the connecting rod blank has been returned to the ambient temperature after splitting.
Further, it is preferable to form, before splitting the connecting rod blank, in the connecting rod blank a connecting hole made up of a threaded hole on a side of the rod main body and a bolt inserting hole which extends from the threaded hole toward the large-end cap and into which is inserted a bolt for connecting the large-end cap. In this case, if the bolt inserting hole is formed larger in diameter than the outer diameter of a tap to be used in tapping of the bolt hole, i.e., larger in diameter than the root diameter of the threaded hole, the play of the bolt relative to the bolt inserting hole becomes large. As a result, the large-end cap is sometimes fastened to the rod main body in a state in which the projections and recessions on the split surface of the large-end cap are engaged with the projections and recessions which are different from those on the split surface of the rod main body. On the other hand, if the bolt inserting hole is formed into a stepped hole having a large-diameter part near an outer end of the large-end cap and a small-diameter part which extends from the large-diameter portion through a stepped part toward the threaded hole, and the diameter of the small-diameter portion is made larger than an outer diameter of the connecting bolt and smaller than a root diameter of the threaded hole, the projections and recessions on the surface of the large-end cap are prevented from deviating to the position facing the projections and recessions that are different from those on the split surface of the rod main body. In this manner, the rod main body can be connected to the rod main body in a state in which the projections and recessions on the split surface of the large-end cap are engaged with the corresponding projections and recessions on the split surface of the rod main body. The accuracy of assembling of the rod main body and the large-end cap can thus be secured.
As the double-action type of forging apparatus, it is preferable to use the one which comprises: an upper die holder for the upper die; a lower die holder for the lower die; a hollow first piston which is inserted into the upper die holder and to which is connected the upper die; a second piston which is inserted into the first piston and to which are connected those large-end portion punch and the small-end portion punch which are provided so as to be movable through the upper die; a third piston which is inserted into the lower die and to which are connected those large-end portion punch and the small-end portion punch which are provided so as to be movable through the lower die; that first oil chamber inside the upper die holder which receives through the first piston a reaction force of closing the upper die toward the lower die; that second oil chamber inside the first piston which is defined between the second piston and a pressure receiving wall on an upper end of the first piston facing the first oil chamber; that air chamber inside the first piston which lie opposite to the second oil chamber with the second piston therebetween; a communicating hole which is formed in the pressure receiving wall and which communicates the first oil chamber and the second oil chamber; that third oil chamber inside the lower die holder which forces the third piston upward; a first joint which is provided in the first piston and which communicates with the second oil chamber; a second joint which is provided in the lower die and which communicates with the third oil chamber and which, at a time of completion of the first forging step, couples with the first joint to bring the second oil chamber and the third oil chamber into communication with each other; and a valve which shuts off the communication between the second oil chamber and the first joint when the downward stroke of the second piston relative to the first piston has exceeded a predetermined value at which the second forging step is completed; wherein the communicating hole is blocked by forcing the second piston against the pressure receiving wall by an air pressure of the air chamber until the first forging step is completed and, after completion of the first forging step, the air chamber is discharged to thereby release the blocking of the communicating hole by the second piston and, after completion of the second forging step, oil is discharged from the third oil chamber.
According to this arrangement, while the communicating hole is blocked by the second piston, that portion of the second piston which receives the oil pressure of the first oil chamber is limited to the portion of the communicating hole. Even if the oil pressure in the first oil chamber increases to the oil pressure corresponding to the load at the time of completion of the first forging step, the second piston can be maintained, by a relatively low air pressure in the air chamber, to the retreated position in which the second piston contacts the pressure receiving wall. Here, as an air pressure source, there can be used an air source which is available inside a plant in which the double-action type of forging apparatus is installed. Therefore, the air source is made available at a low cost.
After the completion of the first forging step, the air is discharged from the air chamber to thereby release the blocking of the communicating hole. Then, oil flows from the first oil chamber into the second oil chamber through the communicating hole. Further, in this state, since the first and second joints are connected together, the oil flows from the second oil chamber also into the third oil chamber through both the joints. By the oil pressures in the second and third oil chambers which operate on the second and third pistons, respectively, each of the punches which are movably provided in the upper and lower dies respectively is pushed into the forming space to thereby perform the second forging step. Then, since a closed oil circuit to communicate the second and third oil chambers together is formed as a result of connection of the first and second joints, it is not necessary to use flexible tubes as the piping material for the closed circuit. The durability can thus be secured.
In a state in which the second forging step has been completed, the communication of the third oil chamber to the second oil chamber is shut off by the valve. Thereafter, by discharging the oil from the third oil chamber, the pushing of the large-end portion punch and the small-end portion punch on the side of the lower die by the oil pressure in the third oil chamber is released. Thus, only the large-end portion punch and the small-end portion punch on the side of the upper die are pushed by the oil pressure in the second oil chamber. In this manner, while pushing the large-end portion punch and the small-end portion punch on the side of the lower die out of the forming space, the large-end portion punch and the small-end portion punch on the side of the upper die are further forced into the forming space, whereby the large-end portion and the small-end portion are pierced or punched with hole, respectively.
It is also possible to close the upper die by moving the upper die holder in interlocking relationship with the press ram. In this case, however, the forging load is limited to the pressing force of the press ram. On the other hand, if a ram piston which moves in interlocking relationship with the press ram is inserted into, or disposed in, the inside of the upper die holder to thereby define the first oil chamber between the ram piston and the first piston, the forging load can be made larger than the pressing force of the press ram, because the diameter of the first piston is made larger than that of the ram piston.
It is considered to form in the lower die a forming hole of the shape which coincides with the contour of the connecting rod blank. In this case, however, the machining work of the lower die becomes troublesome and the cost of the die becomes expensive. It is therefore desirable to make the following arrangement. Namely, on top of the lower die is disposed an intermediate die which is provided with a forming hole of a shape which coincides with the contour of the connecting rod blank. Upon completion of the first forging step, the closed forming space is constituted by the forming hole between the upper die and the lower die. Here, in the intermediate die, there is operated a force, by the forging of the preform, in the first forging step and in the second forging step. The force tries to expand the forming hole in the width direction and in the longitudinal direction. Stresses are likely to concentrate in the periphery of the forming hole, at the following parts or portions, namely, a) on parts coinciding with both side edges on that end of the large-end portion of the connecting rod which lies opposite to the rod portion thereof, b) on parts coinciding with the periphery on both sides having the largest width in the small-end portion of the connecting rod, and c) on a part coinciding with that apex portion of the small-end portion of the connecting rod which lies opposite to the rod portion. Cracks starting from each of the above-described parts a)-c) are thus likely to occur in the intermediate die. In such a case, if the intermediate die comprises: a first segmented die constituting that peripheral part of the forming hole which coincides with such an end in the large-end portion of the connecting rod blank as is opposite to the rod portion of the connecting rod blank; a pair of second segmented dies each constituting that peripheral part of the forming hole which coincides with a section starting from an edge part on each side of the end to a peripheral part on each side of the largest width in the small-end portion of the connecting rod blank; a pair of third segmented dies constituting that peripheral part of the forming hole which coincides with a section from a peripheral part on each side of the largest width to that apex part of the small-end portion of the connecting rod blank which is opposite to the rod portion. Then, the occurrence of cracks due to stress concentration can be prevented and the lifetime of the dies can be prolonged. Further, in order to be sure that no clearance occurs to the divided surface of each of the segmented dies by the expanding force which operates at the time of forging of the preform, the above segmented dies are preferably tightened together by a ring member so that the segmented dies are brought into forcible contact with each other at respective split surfaces. The occurrence of burrs can thus be prevented. In particular, the segmented surfaces of the first segmented dies and each of the second segmented dies as well as the segmented surfaces of the second segmented dies and the third segmented dies are preferably formed such that an angle between a central surface in the widthwise center of the forming hole and each of the segmented surfaces is 45xc2x0. Then, the first through the third segmented dies can advantageously be forcibly pressed against each other by the tightening force from the outer periphery of the ring member.
It is considered to form in each of the upper and lower dies a forming part of projected shape which fits into the forming hole in the intermediate die so that the preform is forged inside the forming hole by this forming part. However, this arrangement has the following disadvantage. Namely, the four corner portions in cross section of the rod portion of the connecting rod blank are formed into the shape of edges, with the result that concentrated stresses are likely to occur. Therefore, the chamfering of the corner portions must be performed in a subsequent step. If an inclined projected edge is formed in the outer periphery of the forming part of each of the upper and lower dies, the corner portion can be formed into a chamfered shape. However, the rigidity of the projected edges cannot be secured and the durability of the dies becomes poor. This solution is therefore not practical. In such a case, preferably, the double action type of forging apparatus comprises: an intermediate die having a forming hole which constitutes a forming space enclosed by the upper die and the lower die upon completion of the first forging step, the forming hole having a hole portion for forming a contour of at least the rod portion, among the contours which are parallel in the direction in which the upper and lower dies face each other; the upper and lower dies each having formed therein: a forming surface including a rod-portion forming-section to form the side surfaces opposing the upper and lower dies; a face surface opposing the intermediate die; and an inclined surface on an outer edge portion of the rod-portion forming-section which chamfers each of the corner portions in cross section of the rod portion. By the inclined surface in the outer edges of the rod-portion forming-section of each of the upper and the lower dies, each corner portion of the rod portion of the connecting rod blank is formed into a chamfered shape. The chamfering work in a subsequent step is not necessary. Here, since there is present the face surface outside the inclined surface, the rigidity of the inclined portion can be sufficiently secured. The durability of the dies will therefore not be lowered. At the time of forging, the preform deforms first at the forging portion by the upper die, and the metal is protruded into the clearance between the face surface of the upper die and the intermediate die, with the result that burrs are likely to occur. In such a case, preferably, the intermediate die is supported through urging means in a manner floated from the face surface of the lower die such that, when the upper die is closed to the lower die, the face surface of the upper die gets seated on the intermediate die, and that the intermediate die gets seated on the face surface of the lower die against the urging means. Then, before the metal protrudes into the clearance between the face surface of the upper die and the intermediate die, the face surface of the upper die gets seated onto the intermediate die. Burrs will thus not occur. Once the intermediate die has seated onto the face surface of the lower die, there is formed a closed space by the upper and lower dies and the intermediate die. The connecting rod blank can thus be formed with a good accuracy.